Reference is made to FIG. 1 showing a prior art ion sensitive field effect transistor (ISFET) 10. The ISFET 10 is provided in and on a semiconductor substrate 12 that is, in this case, doped with a p-type dopant. Source and drain regions 14 and 16, respectively, are provided within the substrate 12, wherein the regions 14 and 16 are doped with an n-type dopant. A gate oxide layer 18 extends on a top surface of the substrate 12 at least over a channel region 20 positioned between the source region 14 and drain region 16. Metal source and drain contacts 22 and 24, respectively, are provided in electrical connection to the source and drain regions 14 and 16. An insulating material 26 covers the structures described above but includes an opening 28 exposing an upper surface of the gate oxide layer 18 above the channel region 20. The opening 26 is configured to receive a test liquid 30. The ISFET 10 does not have a conventional gate electrode. Instead, the deposited test liquid 30 provides the gate electrode. The test liquid 30 is biased by a reference electrode 32 coupled to receive a reference voltage Vref. The test liquid 30 may, for example, comprise a biological or electrochemical material. In operation, the drain current of the ISFET 10 is modulated by the ion content of the test liquid. The ISFET 10 accordingly functions as an ion sensor (for example, a pH sensor).
Reference is made to FIG. 2 showing a prior art ion sensitive field effect transistor (ISFET) 50. The ISFET 50 is provided in and on a semiconductor substrate 52 that is, in this case, doped with a p-type dopant. Source and drain regions 54 and 56, respectively, are provided within the substrate 52, wherein the regions 54 and 56 are doped with an n-type dopant. A gate oxide layer 58 extends on a top surface of the substrate 52 at least over a channel region 60 positioned between the source region 54 and drain region 56. Metal source and drain contacts 62 and 64, respectively, are provided in electrical connection to the source and drain regions 54 and 56. A gate electrode 66 (perhaps made of polysilicon) extends on and over the gate oxide layer 58 along with a metal gate contact 68. An insulating material 70 covers the structures described above with an electrical connection 72 (provided by the metallization layers—metal lines and vias) to the gate contact 68 extending through the insulating material. A conductive chemical gate electrode 74 extends on a top surface of the insulating material 70 and is electrically connected to the electrical connection 72. An insulating passivation layer 78, for example made of oxinitride or silicon nitride, extends on the top surface of the conductive chemical gate electrode 74 is configured to receive a drop of test liquid 82. The test liquid 82 is biased by a reference electrode 84 coupled to receive a reference voltage Vref. The reference electrode 84 may, for example, be deposited on the surface of the insulating passivation layer 78. In this implementation, the electrically connected gate electrode 66 and conductive chemical gate electrode 74 form a floating gate electrode and the liquid deposit provides the sensing gate electrode. The test liquid 82 may, for example, comprise a biological or electrochemical material. In operation, the drain current of the ISFET 50 is modulated by the ion content of the test liquid. The ISFET 50 accordingly functions as an ion sensor (for example, a pH sensor).
See, Al-Ahdal, et al., “High Gain ISFET based vMOS Chemical Inverter,” Sensors and Actuators B 171-171 (2012), incorporated by reference.
See, also, Parizi, et al., “Exceeding Nerst Limit (59 mV/pH): CMOS-based pH Sensor for Autonomous Applications,” IEEE Electron Devices Meeting (2012), incorporated by reference.
See, also, Spijkma, et al., “Beyond the Nerst-limit With Dual-Gate ZnO Ion-Sensitive Field-Effect Transistors,” Applied Physicas Letters 98 (2011), incorporated by reference.